1. Field of Invention
The present invention relates to the field of low-voltage electrical apparatus, and more particularly, to a contact structure of low-voltage electrical apparatus.
2. Related Art
Low-voltage electrical apparatuses may be categorized into distribution electrical apparatuses and control electrical apparatuses in accordance with their position and functions in the electric circuits. Both the distribution electrical apparatus and the control electrical apparatus have a contact system disposed therein. As a fundamental element of a low-voltage electrical apparatus, the contact system directly affects the performance of the apparatus.
Low-voltage circuit breakers are major products in distribution electrical apparatuses. Short-circuit breaking ability is a primary performance index of a low-voltage circuit breaker. When a short-circuit fault occurs in a circuit, the faster a contact of the breaker opens, the better the current-limiting performance is, a better current-limiting performance will greatly reduce the negative effects to the breaker and electrical apparatuses that were caused by short-circuit current and extends the lifetime of the breaker. Contactors are major products in control electrical apparatuses. In addition to frequently closing and breaking rated current, a contactor may also close, break, and carry rated overload current. Therefore, it is desired that a contact of a contactor may bear high mechanical and electrical lifetime. The faster a contact opens, the fewer electric arc is resulted, then fewer burnings is brought to the contact and the lifetime of the contactor may be longer. Control-and-protection switching apparatuses have similar demands on the contact as the breakers when breaking short-circuit current. And, control-and-protection switching apparatuses have similar demands on the contact as the contactors when performing frequent operations.
Contact structures commonly used in the low-voltage breakers have two forms, one is single-breakpoint form and the other is dual-breakpoint form. The dual-breakpoint form further includes translational form and rotational form. A contact structure in a dual-breakpoint translational form will rebound after the contact is repulsed. Therefore, a contact in a dual-breakpoint translational form is usually used in situations with a smaller rated current, or situations that have fewer demands on the protection of short-circuit current. For example, an existing breaker with 32 A or lower rated current has a contact of a dual-breakpoint translational form, and its short-circuit breaking ability (lcs) is generally 50 kA. A breaker with 50 A rated current has a contact structure of a dual-breakpoint translational form, and its short-circuit breaking ability (lcs) is generally 30 kA.
A contactor, as a frequently operated electrical apparatus, has demands on high operating frequency and relatively longer mechanical and electrical lifetime. An actuator of a contactor is usually driven by an electromagnet. Since the contactor itself does not have the ability of breaking short-circuit current, a short-circuit protection apparatus is necessarily disposed in the circuits, and the contactor and the short-circuit protection apparatus needs to be coordinated. When the contact of the contactor is closed, electric arcs generated by contact rebounding will burn the contact and shorter its lifetime. By increasing the pressure of the contact, adverse effects caused by electric repulsion force and the secondary rebounding of the contact may be decreased. However, the increase of contact pressure will increase the attraction force of the electromagnet, and thus will increase the volume of the contactor. Meanwhile, the increase of contact pressure will have increased demands on the mechanical performances of structure members and main springs, resulting in increased product costs.
A control-and-protection switching electrical apparatus, as a multi-functional electrical apparatus, has both short-circuit protection of a breaker and high operating frequency and relatively higher mechanical/electrical lifetime of a contactor. A control-and-protection switching electrical apparatus may be designed integrally or modularly, such a control-and-protection switching electrical apparatus facilitates coordination between a contactor and a short-circuit protection apparatus due to the usage of one set of contactor system and control system. However, the contactor system shall simultaneously meet the needs of both high short-circuit breaking ability of a breaker and high operating frequency and high mechanical, electrical lifetime of a contactor since it shall have functions of both the breaker and the contactor. The design of the apparatus in an existing product uses a contact system similar to a contactor, that is, the contact system utilizes a dual-breakpoint bridge type contact, which results in following adverse effect. When the contacts are repulsed by electric repulsion force generated under large current, the contacts usually rebound quickly, such a condition lasts until the operating mechanism completes a tripping operation. The contacts are damaged with burnings caused by repeated close/open of the contacts, so that its usage lifetime is shortened. Therefore, a set of contact blocking mechanisms shall be specially designed. If the contact system is in single-breakpoint or dual-breakpoint rotational form of a breaker, short-circuit breaking ability may be greatly increased but high operating frequency and high mechanical and electrical lifetime cannot be achieved, and thus cannot meet the overall demand of the control-and-protection switching electrical apparatus. The existing integrally or modularly designed control-and-protection switching electrical apparatus utilizes a contact system of a dual-breakpoint translational form, and has relatively smaller scale capacity. For large-scale products, a combination form of separate components is utilized.